The specific aim of this proposal is the elucidation of the properties of the contractile proteins from animals other than the usual animal, the rabbit. This involves the isolation, purification and physico-chemical description of myosin, actin, actomyosin, paramyosin and tropomyosin as well as an analysis of their interactions at the molecular level to gain insight into the basic mechanisms of contraction and relaxation. An important corollary is their location and arrangement within the myofibril. Through precise knowledge of the ultrastructure of muscle further insight into contraction is gained. Localization of the proteins by antibody techniques and by selective extraction correlated with microscopy, (light, fluorescent and electron) will constantly accompany the biochemical studies. Through a comparative study of muscle from more primitive animals (e.g. the horseshoe crab, clams, etc.) with muscle from rabbit where the bulk of current biochemical work is being done, we hope to shed light on the basic mechanism of contraction in all muscle and, perhaps, in all cells. By sorting those properties which are common to the several muscle types from those which appear special to a particular muscle we should arrive at the point where we will know what to study to solve the problems of muscle contraction. Associated with this gain in knowledge a comparative study also tests the universality of muscle theories and increases general knowledge of different muscles. At the present time we have found the horseshoe crab (Limulus polyphemus) ideally suited to our purpose because 1) it has not changed externally in over 150 million years, 2) it has easily obtainable striated muscle in reasonable quantities, 3) one can perform with it experiments quite comparable to those so easily done with rabbit muscle, and 4) experimental material is cheap, abundant and not too labile.